The c-myc protooncogene plays an important role in the regulation of cellular growth and proliferation. Expression of this important gene is regulated by a number of external stimuli in malignant and nonmalignant cells. We have shown that c-myc expression is selectively inhibited by the G-C specific DNA binding drugs mithramycin. Using this approach we have identified G-C rich sequences located 5' of the P1 and P2 c-myc promoters which bind putative regulator factors. The G-C rich P1 region binds the Sp1 regulatory protein but with G-C rich P2 region does not. We have used double stranded oligonucleotides to the P2 regulatory region to screen a cDNA expression library and have identified and characterized a gene which encodes a protein binding to this region. This proposal will test the hypothesis that the c-myc promoter binding protein (MBP-1) is a c-myc regulatory protein and that it plays an important role in the regulation of c-myc expression. A second c-myc promoter binding protein, MBP-2, which binds to the ME1a2 region of the P2 promoter, has also been identified. Its role in the regulation of c-myc expression will also be characterized. The specific aims of this proposal are: 1. To investigate the role of the MBP-1 and MBP-2 proteins in the regulation of the c-myc promoter activity. 2. To characterize the role of MBP-1 and MBP-2 in the regulation of cellular proliferation. 3. To sequence the MBP-2 gene and to characterize its interaction with the c-myc promoter sequence. 4. To purify and characterize the recombinant and naturally occurring protein products of the MBP-1 and MBP-2 genes and determine their cellular localization. These studies will provide important new information about the regulation of the c-myc protooncogene and will allow us to determine the role of the c-myc promoter binding protein as a putative myc regulatory protein.